This invention claims priority of a German patent application DE 100 21 378.2 which is incorporated by reference herein.
The invention concerns an optical measurement arrangement having an ellipsometer, in which an incident beam of polarized light is directed at an angle of incidence a xcex1xe2x89xa00xc2x0 onto a measurement location on the surface of a specimen, and information concerning properties of the specimen, preferably concerning layer thicknesses, optical material properties such as refractive index n, extinction coefficient k, and the like, is obtained from an analysis of a return beam reflected from the specimen.
Optical measurement arrangements that are based on the principle of ellipsometry or spectrophotometry, and their use for layer thickness measurement, are known in many varieties from the existing art. They have been successfully utilized in particular in the measurement of thin layers on the patterns of wafer surfaces. Whereas an oblique incidence of the measurement light onto the specimen is required in ellipsometry, a perpendicular light incidence is preferable in spectrophotometry in order to rule out polarization effects as much as possible. A measurement arrangement operating on the principle of spectrophotometry is known, for example, from U.S. Pat. No. 5,120,966.
Since increasingly fine patterns and thinner layers are desirable in particular in wafer manufacture, requirements are also increasing in terms of the accuracy of the optical measurement arrangements with which the dimensional consistency of the patterns and layers can be verified.
To allow even complex patterns and layer systems to be measured, for reliable results it is usually necessary to apply several measurement principles; the measurement operations should be performed as rapidly in succession as possible at a single point, since positioning (given that measurement location sizes are on the order of micrometers wide) is very laborious.
Existing measurement arrangements require different optical assemblies for different measurement principles. Arrangement and coordination of the assemblies with respect to one another must be accomplished in such a way that the pertinent beam paths do not, if possible, substantially influence each other. In the case of a measurement arrangement for the inspection of wafer surfaces, for example, a measurement objective of a spectrophotometer must be arranged over the measurement location. It is also necessary to guide the laser beam of a focusing device onto the measurement location so that the region of a specimen to be examined can be correctly positioned with respect to the measurement objective. An additional ellipsometer must then be arranged alongside the measurement objective of the spectrophotometer, and the incident beams of the ellipsometer must also strike the measurement location. A corresponding device of the ellipsometer for collecting and analyzing an output beam of light reflected from the specimen must furthermore be arranged alongside the spectrophotometer measurement objective. The configuration of a measurement arrangement of this kind is, however, relatively complex.
U.S. Pat. No. 5,042,951 describes a measurement arrangement in which ellipsometric measurement can be performed with only one objective. Many different angles of incidence can be analyzed simultaneously, and even a small measurement spot (approximately 1 xcexcm or less) can be used. With the arrangement described therein it is not possible, however, simultaneously to analyze several wavelengths separately and spectroscopically.
U.S. Pat. No. 5,596,406 has made improvements over this; it recommends, inter alia, the simultaneous measurement of several wavelengths using a halogen lamp as the illumination source.
The arrangements proposed in U.S. Pat. No. 5,042,951 and in U.S. Pat. No. 5,596,406 consistently use normal dispersive lens optics and glass-plate beam splitters, however, which are suitable for the VIS-IR region but not for the entire UV-VIS-IR region. The reason is the large chromatic aberration of the specimen image, and the decreasing transmission in the deep UV of broadband antireflection coatings and broadband reflection coatings.
In this context, it is the object of the invention to create an optical measurement arrangement, operating on the principle of ellipsometry, which has a simple, compact configuration and allows a high measurement accuracy, down to the sub-nanometer range, over the entire UV-VIS-IR spectral region.
This object is achieved by an optical measurement arrangement of the kind cited initially in which the incident beam is directed by a mirror objective onto the measurement location on the surface of the specimen, and the return beam is also captured by the mirror objective.
The use of a mirror objective for ellipsometry makes it possible to eliminate the separate optical assemblies hitherto used for the purpose. It is furthermore possible to use the mirror objective simultaneously for spectrography, thus reducing the equipment requirement of the optical measurement arrangement to a single measurement instrument, and allowing a particularly space-saving design for the entire measurement arrangement to be realized.
The mirror objective moreover has the advantage, as compared to optics conventionally used in ellipsometry, of being UV-transparent, so that a measurement with light wavelengths in the spectral region from 190 nm to 800 nm can be performed. In the measurement of small layer thicknesses in particular, measurement with short wavelengths in the UV region results in a high measurement accuracy.
The mirror objective further makes it possible to apply an incident beam onto the measurement location on the specimen within an angular range of 18xc2x0 to 41xc2x0 from the optical axis of the mirror objective. The relatively high numerical aperture of the objective, i.e. its large angle of incidence range, allows both thin and thick layers to be measured with high accuracy. Because of the relatively high aperture of the mirror objective, microspot sizes of approximately 400 nm to 2 xcexcm are possible.
In an advantageous embodiment of the invention, the light reflected from the specimen is introduced via a light-guiding device into an analysis device, the light-guiding device comprising a plurality of individual light-guiding fibers. A further light-guiding device having a plurality of individual light-guiding fibers is provided in order to convey to the analysis device measurement light that is uninfluenced by the specimen. The use of light-guiding fibers permits the analysis device to be arranged very flexibly with respect to the mirror objective and to an illumination source that is necessarily also present. Connecting the light-guiding devices in parallel makes possible a reduction in the occurrence of noise signals upon analysis, since although the measurement signal arrives in noisy fashion at the receiver, that noise is nevertheless correlated with the noise of the reference light channel, so that it can be effectively compensated for.
A polarizing beam splitter is preferably arranged after the mirror objective, in such a way that the return beam coming from the mirror objective is divided, in the polarizing beam splitter, into two s- and p-polarized output beams which are conveyed separately to the analysis device. It is thereby possible to analyze the polarization state of the light reflected from the specimen. The beam splitter can be a Wollaston analyzer or a Rochon analyzer. The Wollaston prism has the advantage over the Rochon prism that the separation angle between the respectively s- and p-polarized output beams is greater.
A focusing lens is preferably arranged between the exit of the polarizing beam splitter and the light-guiding device that is configured in two-channel fashion, in order to focus the s- and p-polarized output light beams obtained from the polarizing beam splitter onto the respective entrances of the channels of the two-channel light-guiding device. The entrances can be arranged at a distance from the analysis device adapted to the physical conditions, and in any orientation with respect thereto. Adaptation to different measurement location sizes, and thus a high light yield, is additionally made possible.
In an alternative embodiment, the mirror objective is adjusted in such a way that it focuses at infinity, so that the return beam coming from the mirror objective is conveyed as an almost parallel light bundle to the polarizing beam splitter arranged between the mirror objective and the analysis device. By way of a lens arrangement placed after the polarizing beam splitter, the still-parallel light beam bundles of the s- and p-polarized output light beams obtained from the polarizing beam splitter are reduced in terms of their beam width so they can be coupled into the entrances of the measurement light guide channels.
In a preferred embodiment of the invention, the individual light-guiding fibers of the light-guiding devices are guided in bundled fashion as far as a coupling apparatus at an entrance of the analysis device, and there spread out. This allows a better separation of the signals from the individual light-guiding fibers to be achieved for subsequent analysis.
The arrangement of the ends of the individual light-guiding fibers can be grouped with respect to one another as necessary, for example by combining the light-guiding fibers of signals of different channels that correspond to one another; or the arrangement of the light-guiding fibers can be accomplished according to the corresponding points on the measurement location. Preferably the light-guiding fibers are sorted according to angle of incidence.
These groupings can be accomplished both mechanically, by combining the individual light fibers, and by way of a subsequent analysis of the arbitrarily spread-out light-guiding fibers by means of a software program, in which the experimentally ascertained positional relationships between the entrance side and exit side are programmed in individually for each light-guiding device.
In a preferred embodiment of the invention, however, the entrance ends and exit ends of the light-guiding fibers have the same positional relationship to one another for each of the light-guiding devices, so that for evaluation of the measurement result, the signals obtained from the different channels possess a similar information structure and thus can easily be compared to one another. If the light-guiding fiber ends that are adjacent at the entrance end are specifically arranged in adjacent formation at the exit end, signal influences between light-guiding fibers extending next to one another can be minimized.
The light-guiding fibers of the individual channels of the light-guiding devices are preferably spread out in a manner adapted to the analysis device used to examine the light, for example to a spectrograph having a slit-shaped entrance and a charge coupled device (CCD) detector for signal acquisition, and in particularly well-organized fashion in linear form.
In a further preferred embodiment, the entrance ends of the light-guiding fibers for each channel of the light-guiding devices are distributed over a surface that corresponds to an opening of an aperture stop within a measurement light beam emitted from the illumination source, the aperture stop being arranged before a beam splitter for splitting the measurement light beam into a component to be influenced by the specimen and a component that is not to be influenced by the specimen. This allows optimum utilization of the light-guiding fibers in the light-guiding devices, as well as a high light yield.
To optimize the light used for measurement, an illumination apparatus having a halogen lamp and a deuterium lamp is provided, the halogen lamp shining through an opening in the deuterium lamp. Also provided, between the illumination device and the aperture stop, is a lens arrangement which sharply images the filament of the halogen lamp in the aperture stop. This results in a homogeneous light-source volume with a broad spectral range from 190 nm to 800 nm that is continuously available for measurement purposes.
To define the specimen field size at the measurement location that is used during measurement, a pinhole mirror, through which the return beam coming from the mirror objective is guided, is provided between the mirror objective and the polarizing beam splitter. The pinhole can be provided, for example, on a semitransparent mirror with which a portion of the light of the return beam can be diverted to a CCD video camera so that a measurement process can be monitored and optionally recorded.
In a further embodiment, the measurement arrangement according to the present invention is equipped with a device for leveling, which is capable of ascertaining and correcting directional deviations between the line normal to the specimen surface and the angle bisector between the incident and return beams of the measurement arrangement. The manner of operation and configuration of such devices is known from the existing art and therefore will not be explained further here.
In the context of the two aforementioned alternative embodiments, an alignable quarter-wave plate of a common type, which allows the entire UV-VIS-IR spectral region to pass, is positioned either directly after the polarizer or directly before the polarizing beam splitter. This advantageously yields the possibility of better control of the polarization state of the light that is to be analyzed, so that a shorter measurement time and greater measurement accuracy can be obtained.
The further advantages of the arrangement according to the present invention substantially consist, in summary, in the fact that short measurement times and thus a higher throughput of, for example, wafers during production inspection can be achieved. In addition, a measurement accuracy of 0.1 nm or less is attained for layer thickness measurements. A substantial advantage is the fact that a complete spectrum in the UV-VIS-IR region can be recorded simultaneously over a wide angle of incidence, thus making possible a high lateral specimen resolution of between 400 nm and 2 xcexcm.
The use of the mirror objective in conjunction with the ellipsometric measurement method allows measurement accuracies in the sub-nanometer range to be achieved with a simple, compact configuration, the incident beam being directed by way of the mirror objective onto the measurement location on the surface, and the return beam also being acquired by the mirror objective. This makes it possible to eliminate the separate optical assemblies used heretofore in the existing art for spectroscopic ellipsometry. This arrangement with a mirror objective can be used for spectroscopic as well as ellipsometric measurements, so that equipment outlay is reduced and a particularly space-saving design for the arrangement as a whole can be realized.
It is possible with this arrangement to determine reflection simultaneously for different stepper wavelengths, such as 193 nm, 250 nm, and 365 nm. Optical material properties, such as, for example, refractive index n and extinction factor can also be measured in time-effective fashion over the very wide UV-VIS-IR spectral region.
The specially configured four-armed light guide cross-section converter that is described herein makes a substantial contribution to achievement of the object of the invention.